The relative equilibrium moisture content of the material often plays a substantial role in the further processing of material widths. In many cases it is sufficient if the equilibrium moisture content is examined in batch quantities by taking samples. For this purpose, commercial moisture measuring devices are offered on the market which operate by a so-called cap sensor. For measuring, the sensor is placed on the material. After a period of adjustment, an equilibrium moisture content is established in the occluded volume of the measuring chamber of the cap sensor and is representative of the product. The surface of the product contributing to the measurement is normally of the order of magnitude of 50 cm.sup.2.
This measuring process is only suitable in the first place for measuring objects with smooth surfaces. In the case of objects with uneven surfaces, the cap sensor does not make flush contact with the surface, thus causing systematic measuring errors. A further source of error in this process is that with very thin material samples, the total available water quantity is not sufficient to produce an equilibrium state in the measuring volume defined by the cap sensor and the measuring cell, without the water content of the sample notably increasing or decreasing. Moreover, the problem exists with this process of sealing the sensor against external air which is produced by the cap sensor being pressed with its own weight onto the sample. Apart from this, fluctuations in temperature also have a damaging influence on the accuracy of the measurement. It is thus to be aimed for that the measurement of the relative equilibrium moisture content of the material sample takes place in a sealed system where disturbances by the surrounding air (air moisture and temperature) can be avoided.